1. Technical Field
The invention is related to RF plasma reactors for processing semiconductor wafers and particularly to improvements therein for increasing the uniformity of plasma ion density across the surface of the semiconductor wafer.
2. Background Art
RF plasma reactors used for semiconductor wafer processing have many applications, including reactive ion etching, sputter etching and chemical vapor deposition, for example. Referring to FIG. 1, an RF plasma reactor specifically designed for reactive ion etching of semiconductor wafers can include a vacuum chamber 100 having a cylindrical side wall 105 and a ceiling 110 supporting a gas distribution plate 115 at the top of the chamber and a conductive wafer pedestal 120 over a base 122 supporting a semiconductor wafer 125 near the middle of the chamber 100, the pedestal 120 being coupled to an RF source 130 through the base 122 to provides the RF energy for the plasma. The gas distribution plate 115 underlies a gas manifold 135 into which gases are introduced through a gas inlet 140. Holes 145 in the bottom of the gas distribution plate 115 distribute gases from the manifold 135 into the interior of the chamber 100, which are ionized by the RF energy from the source 130 to produce the plasma. The ions or radicals in the plasma are attracted toward the wafer 125 by the RF power applied to the wafer pedestal 120. A vacuum pump 150 at the bottom of the chamber 100 removes etchant gases and reactants from the chamber and maintains a desired pressure level therein. An insulator pipe 160 and a conductive outer pipe 165 cover the sides of the wafer pedestal 120 and base 122 while an insulator ring 170 surrounds the edge periphery of the wafer 125.
Etching of Silicon surfaces on the wafer 125 can be accomplished by supplying a mixture of Chlorine (Cl) and Hydrogen Bromide (HBr) gases through the inlet 140. The Chlorine gas disassociates into Chlorine ions and radicals in the plasma which etch Silicon surfaces on the wafer while the Hydrogen Bromide provides reactants in the plasma which attack polymer deposits formed from previously deposited photoresist to regulate the etch profile. The Silicon etch rate is a direct function of the density of Chlorine ions and radicals in the plasma. The pump 150 removes volatile reactants such as Silicon-Chlorine compounds.
A fundamental problem with such a reactor is that the Chlorine ion and radical densities are much greater near the edge periphery of the wafer 125 than over the center thereof, which severely limits etch process yield and integrity. A principal reason for the greater Chlorine ion density near the wafer edge is that the vacuum pump 150 draws the gases introduced from the gas distribution plate 115 at the top of the chamber toward the side wall 105 and therefore away from the center of the chamber, thereby reducing Chlorine ion and radical densities in the region overlying the center of the wafer 125 while increasing Chlorine ion and radical densities in the region overlying the edge periphery of the wafer 125.
A conventional technique for ameliorating this problem has been to provide a vertical ring wall, sometimes referred to as a focus ring, surrounding the edge periphery of the wafer 125 in order to reduce the local Chlorine ion density at the wafer periphery, thereby providing a somewhat more uniform etch rate across the wafer surface. One problem with this approach is that the focus ring tends to trap contaminants near the wafer edge. Since such contaminants can lead to device failure, this technique can lead to a reduction in process yield.
It is a principal object of the present invention to enhance the uniformity of etch rate in an RF plasma etch reactor without requiring a focus ring.
It is a related object of the invention to enhance the uniformity of plasma ion and radical densities across the surface of a semiconductor wafer or workpiece in an RF plasma reactor.